-UPDATE 3/18/13-The box for Sunset Shimmer finally came out. Might redraw this picture now that we have a clearer image of her merchandise vector. Also hints that the rumors about her are more or less false. [link]

-UPDATE 3/19/13-Welp, there you have it. Sunset Shimmer is not going to be in the show afterall! [link]

-UPDATE 5/12/13-SUNSET SHIMMER IS GOING TO BE IN EQUESTRIA GIRLS!! [link] Possibly the main antagonist, too!

My standing theory is that, though she has a nice design, she is actually nopony important. She might make a cameo somewhere, but I doubt her name will even be spoken if she does.

Here is my reasoning.

#1. Her name essentially means "Twilight Sparkle". (Twilight is the time of day when the 'Sun-Set's, and 'Shimmer' is just another word for Sparkle or Twinkle)

I really doubt the writers would make two characters with matching names like that. If they wanted to make a new character, it would not just be a Twilight clone.

#2. Remember that Twilight just became an Alicorn. Think about that from a toy manufacturing perspective. Hasbro want's to release a set of 6 ponies for their masquerade set, and all 6 ponies need to be sold at the same price point. Twilights new Alicorn toy model is now bigger and thus more expensive to manufacture. Thus, rather than sell an Alicorn Twilight figure at regular figure price, they invented a Twilight mirror-clone recolor to take her place in the 6 figure set.

... thus ... she is almost certainly nopony special. (At least until fans make up an epic personality and backstory for her, that is).

And confirmed by show staff that she will NOT be in S4 (or likely ANY season).

I still say she is just a clone toy to take her place in the 6 figure toy set. Her name and description make her the alter-twi ... like alternate timeline Twilight or something ... having nothing to do with the show whatsoever, but instead just being the creation of lazy toymakers who can't come up with an original character personality and so just made another Twilight. ... and caused a bit of fan controversy in the process.

Ah, Sunset Shimmer. The rumored Seventh addition into the Mane Six. Wonder if its true? (Hard to say the only evidence n her appearing in the Masquerade Toy Set and Twilight's newly added sixth white star around the main star of her mark.)

Man, you should see the thoeries I've been hearing, some are even linked with Celestia, Luna and/or the Elements of Harmony. Hopefully we'll get some info soon.

Well there are a few theories to that -One: There's belief that new star marks she fulfilled her destiny completely and her mark was basically incomplete before. Or the rumor of a new Mane Six member coming soon. We barely know anything about the Elements, so there may have very well not originally been just 6 like there are now.

Two: Upon becoming an Alicorn a pony's cutie mark changes. There's no proof their marks do this for this but its been confirmed in the new "Twilight Sparkle and the Crystal Heart Spell" book Cadance was born a regular Pegasus and then became a Alicorn after stopping a pony called Prismia who was being corrupted through an evil necklace and soon after Princess Celestia adopted her so Celestia is actually more so her mother and Luna is her official aunt, and this book was written before the Finale even aired. So its plausible their mark changes after.

Three: Maybe Twilight's mark has more meaning than we think, Lauren Faust stated in the extras of the Brony Documentary she planned for Twilight to succeed Celestia. There's still the possible unknown link between the characters and a possible bigger bad playing the strigngs, perhaps there's a Greater Good doing the same with Celestia and the Mane Six.

Either way, I feel until you can think about a possibility because that's what speculation is. But saying something is true or false isn't possible until you have enough information to tell it is. We all thought the Winged Twilight picture was a joke around the time of Autumn last year even before Season 3 started, but that ended up being the real deal, we basically need to keep our mind open to all possibilities because what we may think something that might not happen may be what happens in the end. Basically "Expect the Unexpected" because it can happen.

Actually we know less about the elements than we think, and remember they were first discovered by Luna and Celestia which were then used to defeat Discord.

What we don't know is:How did they come to exist?Who made them if they were created?What power do they really hold?Why didn't Discord simply destroy them?And most of all, why do they embody Harmony?

Also S3's first episode is when the S3 finale episode was being written, also why would one show her mark with all six stars that early? Because that would be a huge spoiler, and there's plenty of animation errors, AJ sometime doesn't have her freckles for example. In fact there's two YouTube video that is focused only on showing these animation errors for the first and second season (and possibly a third as of now), the writers can miss almost anything when they animate the episode and never realize it.

We don't even know the full story on Celestia, Luna, even villains like Chrysalis, Discord and King Sombra. We know less than we think we know. There's a confirmed season 4 and its heavily hinted that we'll have a season 5. (I have a journal that can show you that, its a very recent one too.)